&nbsp141 &nbspFXUS10 KWNH 191652 &nbspPMDHMD &nbsp &nbspMODEL DIAGNOSTIC DISCUSSION &nbspNWS WEATHER PREDICTION CENTER COLLEGE PARK MD &nbsp1152 AM EST MON FEB 19 2018 &nbsp &nbspVALID FEB 19/1200 UTC THRU FEB 23/0000 UTC &nbsp&nbsp &nbsp..SEE NOUS42 KWNO (ADMNFD) FOR STATUS OF THE UPPER AIR INGEST &nbsp &nbsp&nbsp &nbsp..12Z MODEL EVALUATION WITH PRELIMINARY PREFERENCES/CONFIDENCE &nbsp &nbsp~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ &nbsp &nbsp...AMPLIFIED WESTERN US TROUGH WITH PRIMARY SHORTWAVE LIFTING &nbspNORTHEAST FROM THE FOUR CORNERS TODAY TO ONTARIO BY TUESDAY &nbspNIGHT...&nbsp &nbsp..ASSOCIATED WEAK SURFACE LOW TRACKING INTO THE GREAT LAKES &nbsp &nbsp...TRAILING, SLOW-MOVING FRONT STRETCHING FROM THE GREAT LAKES TO &nbspTHE SOUTHERN PLAINS THROUGH WEDNESDAY... &nbsp~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ &nbspPREFERENCE: NON-NAM BLEND &nbspCONFIDENCE: SLIGHTLY ABOVE AVERAGE &nbsp &nbspFROM A SYNOPTIC SCALE, THE MODELS ARE IN VERY GOOD AGREEMENT ON &nbspTHE BIG PICTURE, WHICH IS TO BE EXPECTED WITH A RELATIVELY STABLE &nbsp1-WAVE PATTERN OVER THE CONUS -- AN AMPLIFIED TROUGH IN THE WEST &nbspAND AN AMPLIFIED RIDGE IN THE EAST. THERE ARE STILL SOME SMALL &nbspDETAIL DIFFERENCES ON THE MESOSCALE, OR WITH TIMING OF INDIVIDUAL &nbspSHORTWAVES EMBEDDED IN THE BROADER FLOW PATTERN, THAT COULD MAKE &nbspAN IMPACT ON THE FORECAST. &nbsp &nbspTHE 12Z NAM KICKS THE PRIMARY SHORTWAVE OUT OF THE BASE OF THE &nbspTROUGH A LITTLE EARLIER THAN THE OTHER MODELS, AND THUS IS ABOUT &nbspSIX HOURS FASTER WITH THE PROGRESSION OF THE WAVE AND THE &nbspASSOCIATED SURFACE LOW. WITH A LACK OF SUPPORT FOR SUCH A SCENARIO &nbspFROM OTHER MODELS OR ENSEMBLE MEMBERS, THE PREFERENCE IS TO &nbspEXCLUDE THE NAM AT THIS TIME. THE 12Z GFS, MEANWHILE, CONTINUES TO &nbspBE ON THE SLOWER END OF THE MODEL SPREAD, WHICH WAS ALSO THE CASE &nbspYESTERDAY. HOWEVER, THE DIFFERENCE BETWEEN THE GFS AND THE OTHER &nbspGLOBAL MODELS HAS NARROWED SUFFICIENTLY, THAT IT IS A REASONABLE &nbspPOSSIBILITY. THE 06Z GEFS WAS SLIGHTLY FASTER WITH THE FORWARD &nbspPROGRESSION OF THE SURFACE LOW, INDICATING THAT THE GFS MAY &nbspCONVERGE FURTHER TO THE OTHER MODELS OVER THE NEXT COUPLE CYCLES. &nbsp &nbspOTHERWISE, THE PREFERENCE IS FOR A BROAD-BASED BLEND TO ACCOUNT &nbspFOR MESOSCALE UNCERTAINTIES. ALONG THE SOUTHERN EXTENT OF THE &nbspFRONT IN PARTICULAR, CONVECTION MAY BE THE PRIMARY DRIVER OF &nbspCERTAIN THINGS, SUCH AS HOW QUICKLY THE FRONT ADVANCES TO THE &nbspSOUTHEAST. A BROADER BLEND WOULD ACCOUNT FOR THE POSSIBILITY OF &nbspSMALL MASS FIELD VARIATIONS HAVING AN EFFECT ON THE SENSIBLE &nbspWEATHER IMPACTS. &nbsp &nbsp...SECONDARY SHORTWAVE DROPPING SOUTH ALONG WEST COAST OF NORTH &nbspAMERICA THROUGH WEDNESDAY, REINFORCING THE LARGER TROUGH OVER THE &nbspWESTERN CONUS... &nbsp...SHORTWAVE KICKING INTO THE FOUR CORNERS ON THURSDAY WITH FRONT &nbspPOSSIBLY RETURNING NORTH ACROSS THE SOUTH-CENTRAL US... &nbsp~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ &nbspPREFERENCE: GENERAL MODEL BLEND &nbspCONFIDENCE: AVERAGE &nbsp &nbspMODELS HAVE COME INTO BETTER AGREEMENT WITH THE SHORTWAVE DIGGING &nbspALONG THE WEST COAST ON TUESDAY AND WEDNESDAY. THE GFS HAD &nbspPREVIOUSLY BEEN MUCH MORE AMPLIFIED WITH THE SHORTWAVE TROUGH &nbspALOFT, AND HAD A MUCH STRONGER SURFACE LOW. WHILE THE OTHER MODELS &nbspHAVE TRENDED SLIGHTLY MORE AMPLIFIED THAN THEY WERE A COUPLE MODEL &nbspCYCLES AGO, THE GFS HAS TRENDED SUBSTANTIALLY LESS AMPLIFIED WITH &nbspTHE TROUGH ALOFT. THE RESULTING MODEL MASS FIELDS ARE RELATIVELY &nbspSIMILAR. THE PRIMARY DIFFERENCE APPEARS TO BE TIMING, WITH THE 12Z &nbspGFS AND NAM SLIGHTLY SLOWER THAN THE 00Z ECMWF, CMC AND UKMET. THE &nbspGFS AND ECMWF BOTH HAVE SUPPORT FROM THEIR RESPECTIVE ENSEMBLE &nbspMEANS. THE DIFFERENCES THROUGH THE SHORT TERM PERIOD WOULD BE &nbspRELATIVELY SUBTLE, BUT A FASTER SOLUTION (CLOSER TO THE ECMWF) &nbspWOULD PRODUCE HEIGHT FALLS OVER THE SOUTHERN PLAINS SOONER, WITH A &nbspCORRESPONDING EARLIER LOW-LEVEL MASS FIELD RESPONSE. THUS, THE &nbspRETURN FLOW AND SURFACE FRONT MAY BEGIN TO PUSH NORTH FROM THE &nbspNORTHWESTERN GULF A LITTLE SOONER. THAT BEING SAID, THERE IS NOT A &nbspCLEAR PREFERENCE BETWEEN THE SMALL TIMING DIFFERENCE, SO A GENERAL &nbspMODEL BLEND IS PREFERRED WITH THIS SYSTEM. &nbsp &nbsp...THIRD SHORTWAVE ALSO DIGGING SOUTH ALONG WEST COAST WEDNESDAY &nbspNIGHT AND THURSDAY AND FURTHER REINFORCING THE TROUGH... &nbsp~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ &nbspPREFERENCE: BLEND OF 12Z NAM, 06Z GEFS, 00Z ECMWF &nbspCONFIDENCE: SLIGHTLY BELOW AVERAGE &nbsp &nbspMORE SIGNIFICANT DIFFERENCES EMERGE WITH THE MID-WEEK SHORTWAVE &nbspDIGGING INTO THE WEST COAST. THE 12Z GFS DIGS THE PRIMARY &nbspSHORTWAVE INTO SOUTHERN CALIFORNIA AND HAS THE PRIMARY MID-LEVEL &nbspVORTICITY MAXIMUM IN THAT AREA BY 23/00Z. THE OTHER MODELS HAVE IT &nbspFURTHER NORTH, AND THIS IS SUPPORTED BY THE VAST MAJORITY OF THE &nbspGEFS/ECMWF ENSEMBLE CLUSTERS. THE 06Z GEFS MEAN HAS LOWER HEIGHTS &nbspNEAR WA/OR, WITH HIGHER HEIGHTS IN THE DESERT SOUTHWEST, SO THE &nbspGEFS MEAN SEEMS LIKE A REASONABLE ALTERNATIVE TO THE GFS. THE 00Z &nbspCMC CLOSES OFF AN UPPER LEVEL LOW VERY QUICKLY AND HAS A MUCH MORE &nbspCONCENTRATED VORTICITY MAXIMUM RELATIVE TO THE OTHER MODELS (WHICH &nbspHAVE AN ELONGATED/SHEARED APPEARANCE ALONG THE BASE OF THE &nbspTROUGH). THESE DIFFERENCES IN THE GFS AND CMC SEEM SUBSTANTIAL &nbspENOUGH TO NOT BE INCLUDED IN THE PREFERENCE AT THIS TIME. &nbspTHEREFORE, WILL LEAN MORE IN THE DIRECTION OF A BLEND OF THE &nbspECMWF, NAM AND GEFS MEAN. &nbsp &nbsp...LARGE MID-UPPER LEVEL ANTICYCLONE JUST OFF THE SOUTHEAST COAST &nbspWITH EAST COAST RIDGE AXIS... &nbsp~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ &nbspPREFERENCE: BLEND OF 00Z ECMWF, ECMWF ENS MEAN &nbspCONFIDENCE: AVERAGE &nbsp &nbspTHE 00Z CMC AND UKMET HAVE ONE OF THE STRONGER DEPICTIONS OF THE &nbspUPPER LEVEL RIDGE IN THE FIRST 24 HOURS OF THE FORECAST, AND THIS &nbspEVENTUALLY GROWS OVER TIME BY MID-WEEK. THE 00Z UKMET EVENTUALLY &nbspHAS HIGHER HEIGHTS ALOFT FROM THE TENNESSEE VALLEY INTO THE UPPER &nbspMIDWEST, AND HAS THE BIGGEST DEVIATION FROM THE MODEL AVERAGE. &nbspMEANWHILE, THE 12Z GFS IS MUCH FLATTER WITH ITS DEPICTION OF THE &nbspRIDGE BY WEDNESDAY AND THURSDAY, AND HAS LOWER HEIGHTS OVER MUCH &nbspOF THE NORTHEAST. THE PREFERENCE IS FOR A DEPICTION IN THE MIDDLE &nbspOF THESE EXTREMES, AND CLOSEST TO A BLEND OF THE 00Z ECMWF AND 00Z &nbspECMWF ENSEMBLE MEAN. &nbsp &nbspMODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML &nbsp500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML &nbsp &nbspLAMERS &nbsp &nbsp &nbsp